Pulse-forming network for a short pulse linear accelerator

ABSTRACT

A pulse-forming network is disclosed as employed for pulsing the beam of a microwave linear accelerator to provide a train of beam pulses, such pulses being on the order of 2 nanoseconds long. The pulse-forming network includes a pair of open-circuited coaxial cables connected in parallel between a source of potential and an adder. The adder supplies a pulse output to the cathode of the electron gun of the accelerator. One of the parallel branches of the pulse-forming network includes a series connection of one of the coaxial cables, and a delay, and a pulse inverter. The inverted and noninverted pulses obtained by simultaneously shorting the charged coaxial lines to ground are added in the adder to produce a resultant negative pulse applied to the cathode of the gun for turning the gun on for the short 2 nanoseconds. Ferrite-loaded coaxial pulse shapers are disposed in each of the parallel branches for sharpening the leading edge of the resultant negative pulse. One of the distributed coaxial lines is substantially longer than the other to make the positive inverted pulse longer than the negative noninverted pulse, such that the gun is held in the OFF condition for a period following each of the pulses to prevent unwanted dark current. A coupling capacitor is connected between the pulse inverter and ground for holding off a positive quiescent DC bias potential applied to the cathode of the gun. A variable DC current is fed through one of the pulse shapers for varying the pulse width of the resultant pulse.

nite States atet [72] inventor DennisE. Morris Sunnyvale, Calif.

21 Appl. No. 803,921

[22] Filed Mar. 3, 1969 Continuation-in-part of Ser. No. 792,709, Jan 21, 1969.

[45] Patented May 25, 1971 [73] Assignee Varian Associates Palo Alto, Calif.

[54] PULSE-FORMING NETWORK FOR A SHORT PULSE LINEAR ACCELERATOR Primary ExaminerRodney D. Bennett, Jr.

60 FT. 0F RG58 ISONS.

Assistant Examiner-Joseph M. Potenza AttorneysLeon F. Herbert and Stanley Z. Cole ABSTRACT: A pulse-forming network is disclosed as employed for pulsing the beam of a microwave linear accelerator to provide a train of beam pulses, such pulses being on the order of 2 nanoseconds long. The pulse-forming network includes a pair of open-circuited coaxial cables connected in parallel between a source of potential and an adder. The adder supplies a pulse output to the cathode of the electron gun of the accelerator. One of the parallel branches of the pulseforming network includes a series connection of one of the coaxial cables, and a delay, and a pulse inverter. The inverted and noninverted pulses obtained by simultaneously shorting the charged coaxial lines to ground are added in the adder to produce a resultant negative pulse applied to the cathode of the gun for turning the gun on for the short 2 nanoseconds. Ferrite-loaded coaxial pulse shapers are disposed in each of the parallel branches for sharpening the leading edge of the resultant negative pulse. One of the distributed coaxial lines is substantially longer than the other to make the positive inverted pulse longer than the negative noninverted pulse, such that the gun is held in the OFF condition for a period following each of the pulses to prevent unwanted dark current. A coupling capacitor is connected between the pulse inverter and ground for holding off a positive quiescent DC bias potential applied to the cathode of the gun. A variable DC current is fed through one of the pulse shapers for varying the pulse width of the resultant pulse.

';/ ACCELERATOR TRIGGER. PULSE DELAY l2 FT. RG58 PATENTEU HAYES 197i 60 FT. 0F RG58 ISONS. I a 22- 9 (INVERTER l3 FIIGJI FIG.3

ISONS.

IONS.

TIM INVIIN'TUR. DENNIS E. MORRIS PULSE-FORMING NETWORK FOR A SHORT PULSE LINEAR ACCELERATOR The invention described herein was made in the course of,

or under Subcontract 5603 under Contract W-7405-Emg-26 with the US. Atomic Energy Commission. The present application is a continuation-in-part application of US. Pat. application Ser. No. 792,709 filed Jan. 21, 1969 assigned to the same assignee as the present invention.

DESCRIPTION OF THE PRIOR ART Heretofore, pulse generators have been employed for pulsing the cathode of the electron gun of a microwave linear accelerator for producing a pulsed accelerator beam. However, such prior pulse networks have included a distributed transmission line element arranged to be periodically discharged to produce a relatively sharp output pulse which is applied to the cathode of the gun. However, such prior pulse forming generators have not been capable of producing pulses on the order of a few nanoseconds long with the relatively high voltages, as of4 kv., required to produce extremely short high peak power pulses of beam current.

It is also known from an article titled Generator of High Voltage Nanosecond Pulses" appearing in the Russian journal, Pribory l Tekhnika Eksperimenta, No. 5, pp. 1 36- 1 39, of the September-Oct. I965 issue, to generate high voltage nanosecond pulses by combining the pulses of a parallel connection of coaxial lines. More particularly, the prior pulse generator employed two open-circuited coaxial transmission lines connected in parallel with a source of positive potential. The parallel connection of transmission lines was periodically short-circuited to ground to cause displacement current pulses to flow in the parallel networks connected to the transmission lines. One of the networks included a delay and an inverter and both parallel connections included ferrite-loadedsections of coaxial line for sharpening the pulses. The inverted and noninverted pulses were combined in an adder circuit to produce a very short pulse on the order of a few nanoseconds, with the pulse width being determined by the amount of delay and by a bias current fed through the coaxial line to change the magnetization of the ferrite loading members. Such a pulse generator generated pulses at a repetition frequency of to 1,000 Hz. with an amplitude of 4 kv. with a pulse width from 2 to 100 nanoseconds, and a rise and fall time less than I nanosecond across a 75-ohm load. However, these prior pulse generators have not, heretofore, been employed for pulsing the beam current of a microwave linear accelerator.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved pulse network for linear accelerators.

One feature of the present invention is the provision, in a pulse network for pulsing a linear accelerator, of a pair of distributed transmission lines which are periodically charged and discharged to produce a pair of pulses in parallel networks having a delay and an inverter in one of the parallel branches for inverting one of the pulses which is then combined with a noninverted pulse from the other line to produce a resultant negative pulse which is applied to the cathode of the gun for the accelerator to produce a beam having extremely short beam pulses.

Another feature of the present invention is the same as the preceding feature including the provision of a positive source of DC potential for biasing the cathode of the gun to cutoff potential in the absence of a pulse such that no beam current flows in the quiescent state, and the additional provision of a capacitor connected between the pulse inverter and ground for isolating the inverter from ground potential for the D.C. bias potential applied to the cathode electrode.

Another feature of the present invention is the same as any one or more of the preceding features wherein the distributed line which produces the inverted pulse is longer than the distributed line which produces the noninverted pulse such that the electron gun is biased to an OFF condition during a period following each of the resultant negative pulses.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagram, partly in block diagram form, of a linear accelerator employing the pulse forming network of the present invention,

FIG. 2 is a plot of voltage versus time depicting the pair of pulses which are combined in the adder circuit to produce the resultant pulse applied to the gun of the accelerator,

FIG. 3 is a plot of voltage versus time depicting the resultant negative pulse-to be applied to the cathode of the accelerator,

FIG. 4 is an enlarged plot of voltage versus time depicting the voltage applied to the cathode of the gun in the accelerator, and

FIG. 5 is a plot of beam current I, versus time depicting a current pulse generatedby the voltage pulse of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown the linear accelerator I with the pulse-generating network 2 for producing a train of nanosecond high voltage pulses applied to the cathode of the gun of the accelerator. First and second lengths of distributed transmission line 3 and 4, respectively, such as 60- and 30-foot lengths of RG 58 coaxial cable. The center conductors of cables 3 and 4 are connected in parallel and to a source of high positive potential at terminal 5 via lead 6 and isolating resistor 7. The transmission lines 3 and 4 are connected in series with two parallel networks 9 and 8, respectively. Parallel network 8 includes a ferrite-loaded section of coaxial line forming a pulse shaper 11 for sharpening the leading edge of the pulses generated in network 8. The construction of a suitable shaper is described in the aforecited Russian article.

The second parallel network 9 includes a series connection of a delay 12, an inverter 13, and a second pulse shaper 14. The delay comprises, for example, a 12-foot length of RC 58 coaxial cable and the inverter 13 comprises, for example, two sections of R6 58 coaxial cable with the center conductor of the first section being connected to the outer conductor of the second section and the outer conductor of the first section being connected to the center conductor of the second section.

A thyratron IS a connected between lead 6 and ground for periodically shorting the center conductors of the transmission lines 3 and 4 to ground through the thyratron in response to trigger pulses applied to the control electrode of the thyratron 15. When the transmission lines 3 and 4 are discharged through the thyratron 15, a displacement current flows in the parallel branches 8 and 9. At the output of the inverter 13 the current pulse flowing in branch 9 will be of the opposite sign to the current pulse flowing in branch 8. The output current pulses of branches 8 and 9 are combined at the cathode of the gun of the accelerator, indicated schematically by load resistor 16 which represents the impedance of the gun. When the center conductors of lines 3 and 4 are chargedwith a positive potential, as applied to terminal 5, the voltage pulse output of the inverted branch 8 will be negative as indicated by pulse 18 of FIG. 2. Under the saMe conditions, the inverted voltage pulse is positive, as indicated at 19 in FIG. 2. The positive inverted pulse 19 is delayed relative to the noninverted pulse 18 via delay I2 to produce a resultant negative pulse 21 indicated in FIG. 3. The resultant negative pulse is is applied via lead 22 to the thermionic cathode electrode 23 of the electron gun of the microwave accelerator I.

A positive bias potential as of +2 kv. is applied continuously to the cathode electrode 23 for holding the electron gun in the OFF condition during the quiescent period between pulses. The bias potential is applied via source 24 connected between cathode electrode 23 and ground G or the pulser. An inductive choke 25 is connected between the source 24 and ground G to prevent the pulses 21 from being shunted through the source 24 to ground G,.

A relatively large R.F. coupling capacitor 27 is connected between the outer conductor of the first section of the pulse inverter 13 and ground G for permitting proper operation of the pulse inverter while holding off the DC positive bias potential applied to the cathode electrode 23 via source 24. A typical coupling capacitor 27 is a 0.5 microfarad capacitor at 5 kv. voltage.

A typical composite voltage, as seen at the cathode electrode 23, is depicted in FIG. 4. More particularly the cathode potential, in the quiescent state, is at +2 kv. relative to a control grid 26 which is connected to pulser ground G and operated'at l50 kv. relative to the anode 30 of the electron gun of the accelerator 1, such anode 30 being operated at the accelerator ground potential of zero volts. The resultant negative pulse 21, which has an amplitude of approximately 4 kv., drops the potential at the cathode 23, within approximately 2 nanoseconds, from +2 kv. to 2 kv. relative to the potential of control grid 26. The delay 12 is adjusted and the pulse shapers l l and 14 are adjusted so that the leading and trailing edges of the resultant pulse are as steep as possible with an extremely short pulse width as of 2.3 nanoseconds, between -l kv. points.

The advantage of applying the pulses to the cathode 23, instead of to the control grid 26, is that the cathode 23 has a low impedance which is on the order of 50 ohms such that 50 ohms coaxial line elements may be employed in the pulser. A lower impedance pulse system has less tendency to ring after each pulse. MOreover, pulsing the cathode 23 permits use of negative pulses which are easier to generate than positive pulses. A suitable cathode 23 and control grid 26 comprises a spherically concave cathode emitter having a closely packed array of spherically concave cathode emitter having a closely packed array of spherical dimples disposed in registration with apertures in a spherically concave grid 26 to define a multiplicity of Pierce-type beamlets. Such an electron gun is disclosed and claimed in copending US. Pat. application Ser.

No. 650,893 filed July 3, 1967 and assigned to the same assignee as the present invention.

Referring now to H0. 5, there is shown the typical beam pulse produced by the voltage pulse of FIG. 4. The typical beam pulse has a triangular shape with a peak current of approximately 30 .amperes and a half-height width of approximately 2.3 nanoseconds.

The shape of the resultant negative pulse 21 is controlled by varying a bias current through the center conductor of the ferrite-loaded pulse shapers 11 and 14. One of the bias circuits is shown connected in parallel with a portion of parallel network 9 and comprises a current generator 31 producing a current variable from to amps at approximately 5 volts. The current generator 31 is connected in circuit with the center conductor of the ferrite-loaded coaxial cable shaper 14 via the intermediary of the pair of R.F. chokes 32, as of 200 microhennes.

The transmission line section 3 which produces the positivegoing pulse is made substantially longer than the other transmission line section 4 such that the positive-going pulse 19 has a pulse length substantially longer than that of the negative pulse 18; for example, pulse 19 may have a pulse length of approximately 150 nanoseconds, whereas pulse 18 has a pulse length of, for example, 70 nanoseconds. The longer positivegoing pulse assures the electron gun will be held in an OFF condition for a certain period as of 70 nanoseconds after the termination of the negative pulse 18. If both transmission lines 3 and 4 were of the same length, or nearly the same length, termination of the positive-going pulse 19 before termination of the negative-going pulse 18 would result in a negative pulse which could inadvertently turn the gun ON to produce pulse instability. in a typical example of the pulse network 2, trigger pulses are applied to the thyratron 15 at a maximum pulse re etition rate of approximately 1 kilohertz.

ince many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings celerator structure, means forming a pulse network connected to said cathode electrode for pulsing said cathode electrode negative with respect to said anode electrode for producing pulses of beam current, the improvement wherein, said pulse network includes first and second distributed transmission lines connected in first and second parallel networks, respectively, between a source of potential and said cathode electrode for charging said transmission line to produce stored energy therein, means for periodically shorting said transmission lines to ground for discharging said transmission lines and for producing a pair of pulses in said first and second parallel networks, one of said parallel networks including a delay means and a pulse inverter means for inverting and delaying the pulses in one of said parallel networks relative to the pulses in the other network, means for adding the inverted pulses and the noninverted pulses to produce a train of resultant pulses which are applied to said cathode electrode, said resultant pulses having a pulse width determined by the extent of the delay produced by said pulse delay means, means for applying a DC positive potential to said cathode electrode for biasing off said electron gun in the quiescent state, and means forming a capacitor connected between said pulse inverter and ground for isolating said inverter from ground potential and for holding off the DC positive bias potential applied to said cathode electrode.

2. The apparatus of claim 1 wherein said distributed transmission line which produces the pulse which is delayed by said delaying means is substantially longer than the other distributed transmission line, whereby the delayed pulses are substantially longer than the nondelayed pulses to assure that said gun is biased off during a period following each of the resultant negative pulses.

3. The apparatus of claim 2 wherein each of said distributed transmission lines comprise lengths of coaxial cable open circuited at one end.

4. The apparatus according to claim 1 including means forming a ferrite-loaded pulse shaper in said parallel network which includes said pulse inverter, means for generating and passing a bias current through said ferrite-loaded pulse shaper for shaping the trailing edge of the resultant negative pulse. 

1. In a pulse network for pulsing the beam of a linear accelerator, means forming an electron gun structure having a cathode electrode and an anode electrode for forming and projecting a beam of electrons through a microwave linear accelerator structure, means forming a pulse network connected to said cathode electrode for pulsing said cathode electrode negative with respect to said anode electrode for producing pulses of beam current, the improvement wherein, said pulse network includes first and second distributed transmission lines connected in first and second parallel networks, respectively, between a source of potential and said cathode electrode for charging said transmission line to produce stored energy therein, means for periodically shorting said transmission lines to ground for discharging said transmission lines and for producing a pair of pulses in said first and second parallel networks, one of said pArallel networks including a delay means and a pulse inverter means for inverting and delaying the pulses in one of said parallel networks relative to the pulses in the other network, means for adding the inverted pulses and the noninverted pulses to produce a train of resultant pulses which are applied to said cathode electrode, said resultant pulses having a pulse width determined by the extent of the delay produced by said pulse delay means, means for applying a DC positive potential to said cathode electrode for biasing off said electron gun in the quiescent state, and means forming a capacitor connected between said pulse inverter and ground for isolating said inverter from ground potential and for holding off the DC positive bias potential applied to said cathode electrode.
 2. The apparatus of claim 1 wherein said distributed transmission line which produces the pulse which is delayed by said delaying means is substantially longer than the other distributed transmission line, whereby the delayed pulses are substantially longer than the nondelayed pulses to assure that said gun is biased off during a period following each of the resultant negative pulses.
 3. The apparatus of claim 2 wherein each of said distributed transmission lines comprise lengths of coaxial cable open circuited at one end.
 4. The apparatus according to claim 1 including means forming a ferrite-loaded pulse shaper in said parallel network which includes said pulse inverter, means for generating and passing a bias current through said ferrite-loaded pulse shaper for shaping the trailing edge of the resultant negative pulse. 